Hi Justine. My son did not get seasick on his first cruise, even though we hit choppy seas on our way back from Bermuda and many of the passengers were affected (including me). It could just be him, though, as he has never gotten seasick on any of his 15 cruises since then.
I'd advise checking with your son's pediatrician about what to do should it happen. I have found that having an apple works for me. If that's something he can eat, it might be worth a try.
I wish you and your family a wonderful cruise. Take lots of photos.